1. Field of the Invention
The present invention relates generally to a process for the fabrication of detail parts for superconducting magnets by resin transfer molding and also to the subsequent fabrication of the superconducting magnet. More particularly, the subject invention pertains to a resin transfer molding process for the fabrication of detail parts, such as end saddles, wedge tips, spacers and keys, which are utilized in the assembly of a superconducting magnet.
The construction and fabrication of superconducting magnets such as are utilized in the Relativistic Heavy Ion Collider (RHIC) requires that the coils for such superconducting magnets be fabricated to close tolerances and with materials which will function at relatively high radiation levels and at cryogenic temperatures. During a coil winding operation for a superconducting magnet, precise placement is required of detail parts such as wedge tips and end saddles, particularly the ruled surfaces thereof, and also spacers and keys. Moreover, the dimensions of the coil assembly after winding are considerably larger in length and azimuthal size than the final dimensions of a finished coil assembly, and the coil assembly after winding is subjected to compression at an elevated temperature in a curing press to produce a finished coil assembly.
The location of each wedge tip and end saddle, especially the ruled surfaces thereof, in particular affects how the superconducting coil functions, and there cannot be any voids in the finished coil as void areas generate heat and cause the magnet to quench. When this occurs, the entire superconducting ring must be returned to ambient temperature and the magnet with the void must be removed and replaced. This corrective action is very expensive and time-consuming. During fabrication of the coil winding assembly, properly locating ruled surfaces of the wedge tip and end saddle to the coil windings is critical to avoiding any voids in the finished superconducting coil.
Presently, end saddles are machined from: G-11CR material and wedge tips are machined from G-10CR epoxy material. The dimensions and curves of an end saddle and wedge tip are critical, and require machining of the G-11CR or G-10CR epoxy material in a five axis machining operation which is quite complex. Additionally, machining the end saddles and wedge tips induces internal stresses therein which distort the part's geometry. Moreover, these materials are not elastic, which requires that the parts be machined to final dimensions. Also, the end saddles and wedge tips are positioned on a coil winding which does not achieve its final dimensions until the completion of the curing process. Generally, producing these components by machining as described results in a high scrap rate and elevated costs.
Resin Transfer Molding (RTM) is a well known efficient and very flexible (in terms of component design) fiber reinforced plastics (FRP) manufacturing process. The RTM process involves creating a composite, a combination of two or more materials.
One material is a matrix, most often a thermosetting resin which takes the shape of the finished part and provides color and surface finish therefor. The resin system is generally selected by the molder depending on the specific chemical, electrical, mechanical or thermal properties required for the finished part. Additives and special compounds are available to provide further properties such as surface finish, flame retardance, weather resistance, degree of shrinkage, rate of gel and cure, etc.
The second part of the composite is a reinforcement material, usually in the form of a polyester material or a glass fiber material, but the reinforcement material may include high specific strength materials such as KEVLAR glass fabric material and carbon fiber, as well as various types of core materials and inserts. The reinforcement provides the composite part with strength and toughness. Dry fiber reinforcement in many forms applicable to RTM are placed in the mold prior to clamping and injection. Preforms are dry, preshaped fiber structures (engineered birds nests) resembling the final molded component minus the matrix, and offer an attractive alternative to the slow and laborious manual "floppy fiber mat" loading.
The strength-to-weight ratios of well designed composite structures can be many times that of high tensile steel. Furthermore, it is possible to tailor the properties of a composite structure to exactly match product design requirements.
In its simplest form, the RTM process is a method of molding components from fiber reinforced resins in a two-piece matched cavity mold using pressure. While the RTM mold is open the reinforcing material in dry form is loaded into one of the halves, usually the female. The two halves are then closed and the mold either manually or hydraulically clamped shut.
At this point, a live or catalyzed resin is injected under pressure into the fiber loaded cavity. Depending on the reaction or gel time of the resin and whether the molds are internally heated to assist the reaction process, the mold halves can be opened after a short curing time and the finished part removed. The RTM mold is now ready to be recycled in another molding operation as described hereinabove.